Evaluation of major genetic loci contributing to inbreeding depression forsurvival and early growth in a selfed family of Pinus taeda

The magnitude of fitness effects at genetic loci causing inbreeding depress ion at various life stages has been an important question in plant evolutio n. We used genetic mapping in a selfed family of loblolly pine (Pinus taeda L.) to gain insights on inbreeding depression for early growth and viabili ty. Two quantitative trait loci (QTLs) were identified that explain much of the phenotypic variation in height growth through age 3 and may account fo r more than 13% inbreeding depression in this family. One of these QTLs map s to the location of cad-nl, a lignin biosynthesis mutation. Both QTLs show evidence of overdominance, although evidence for true versus pseudo-overdo minance is inconclusive. Evidence of directional dominance for height growt h was noted throughout the genome, suggesting that additional loci may cont ribute to inbreeding depression. A chlorophyll-deficiency mutation, spf; di d not appear to be associated with growth effects, but had significant effe cts on survival through age 3, Previously identified embryonic viability lo ci had little or no overall effect on germination, survival, or growth. Our results challenge, at least in part, the prevailing hypothesis that inbree ding depression for growth is due to alleles of small effect. However, our data support predictions that loci affecting inbreeding depression are larg ely stage specific.